Alternate flood process for recovering petroleum

ABSTRACT

The specification discloses a process for recovering petroleum from a subterranean reservoir. Gas is injected into the reservoir to establish a free-gas phase in at least a portion of the reservoir. Gas and water are then injected simultaneously into the reservoir and fluids are produced therefrom. The alternate gas and simultaneous gas-water injection steps are repeated until production drops below an economic level.

L1)L) J,J 31" P197912 GR 395999717 m I- m U IHlEQfl mates raw! [72] Inventor James M. McMillen Arlington, Tex. [2]] Appl. No. 881,871 [22] Filed Dec. 3, 1969 [45] Patented Aug. 17, I971 [73] Assignee Mobil Oil Corporation [54] ALTERNATE FLOOD PROCESS FOR RECOVERING PETROLEUM 7 Claims, No Drawings [52] US. Cl 166/273,

166/274 [51] Int.Cl ..E21b43/22 [50] Field of Search [66/273, 274, 263

[56] References Cited UNITED STATES PATENTS 2,875,831 3/1959 Martin et al. 166/274 X 3,065,790 11/1962 Holrn 166/274 3,080,917 3/1963 Walker 166/273 3,084,743 Q4/1963 West et al. 166/273 3,123,134 3/1964 Kyte et al..... 166/274X 3,244,228 4/1966 Parrish 166/274 X 3,472,320 10/1969 Dyes 166/273 Primary Examiner-Stephen J. Novosad AttorneysWilliam J. Scherback, Frederick E. Dumoulin,

Drude Faulconer, Andrew L. Gaboriault and Sidney A. Johnson ALTERNATE FLOOD PROCESS FOR RECOVERING PETROLEUM 1 BACKGROUND OF THE INVENTION The present invention relates to a' process for recovering petroleum form subterranean formations and more particularly to an improved process of injecting gas into a reservoir and following the gas injection with a simultaneous gas-water flood for the recovery of petroleum or like minerals therefrom.

It is well known in the oil industry that so-called secondary recovery processes are employed to produce additional volumes of gas and oil from a subterranean reservoir after production by primary means has declined to an uneconomical level. The more commonly known secondary recovery procedures involve injecting drive fluids such as gas or water into a partially depleted reservoir through an injection system to drive oil or the like toward a production system from which the oil is produced along with portions of the driving fluid. When the ratio of driving fluid to oil reaches an uneconomical level, the reservoir is normally abandoned even though a substantial amount of residual oil still remains in the reservoir.

It has recently been recognized that, in some instances, greater recovery efficiencies can be obtained by flooding with both gas and water in a single recovery procedure. These procedures utilize the injection of gas into a reservoir to establish a free-gas phase within the reservoir. It is believed that this gas occupies space within the reservoir which would otherwise be occupied by trapped oil thereby reducing the amount of residual oil in the reservoir. One such procedure is disclosed in U.S. Pat. No. 3,123,134, issued Mar. 3, 1964, and involves injecting gas into a watered-out reservoir in quantities and at a pressure sufficient to establish a free-gas phase therein. The gas is normally injected through injection wells while maintaining a suitable back pressure on production wells until the desired gas phase is established in the reservoir.

Then the injection wells are closed and the production wells.

are allowed to flow freely. After substantially no more oil can be recovered or the production rate has declined to an undesirably low level, the process is repeated by reestablishing the high-pressure, free-gas phase and again reducing the reservoir pressure opening the production wells. This is continued until production becomes uneconomical. In this process, no additional water is injected into the reservoir after the initial waterflooding operations but, instead, gas is injected as the sole driving medium,

Another alternate injection recovery procedure in U.S. Pat. No. 3,244,228, issued Apr. 5, 1966, and involves alternately flooding the reservoir with gas and water. Gas is injected into a watered-out reservoir or the waterflooded portion of an existing waterflooding operation until a desired gas saturation exists in the reservoir. Water is then injected and the alternate gas-water injection cycles repeated until the water-oil ratio becomes uneconomical. However, even with alternate gas-water flooding, such as described above, significant residual oil still remains in the reservoir at the completion of such processes.

In still other alternate flood procedures, small slugs of gas have been alternately injected with small slugs of water. Though the water and gas are alternately injected into the reservoir, the manner in which they are injected allows the two phases to mix with one another almost as soon as they enter the formation. Since the first few feet around the wellbore serve as a mixing zone, the bulk of the reservoir is subjected to the simultaneous flow of gas and water. Little or no trapped gas remains in any significant portion of the reservoir and hence any benefits derived from a free-gas phase are lost.

BRIEF SUMMARY OF THE INVENTION a free-gas phase is established in the reservoir before the reservoir is subjected to a simultaneous gas-water flood.

is disclosed In carrying out the present invention, gasis injected into the reservoir through an injection system until a free-gas phase-is established in at least a portion of the reservoir. The amount of gas injected will depend upon the size and physical characteristics of the reservoir. Once this free-gas phase is established, gas and water are then simultaneously injected into the reservoir through the injection system while fluids are produced from the reservoir through a production system. During the simultaneous injection step, the gas and water mixture may vary from 25 to 75 percent as much gas, measured at reservoir conditions, as water. The gas injection step and the simultaneous gas-water step may be alternately repeated until production drops below an economical level. The actual operation and apparent advantages of the invention will be better understood by referring to the following detailed description.

DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention provides an improved recovery process for recovering petroleum form a recovery zone of an oil-bearing reservoir. As will be understood by those skilled in the art, the term recovery zone as used herein means that portion of a reservoir through which oil is displaced from an injection system to a production system by an injected fluid. The injection and production systems may comprise one or more wells extending from the surface of the earth into the recovery zone and such wells may be located and spaced from one another in any desired pattern. For example, the so-called line flood" pattern may be utilized, in which case the injection and production systems comprise rows of wells spaced from one another. In this type pattern the recovery zone as definedby the spaced rows of injection and production wells generally will be that portion of the reservoir underlying the area between these spaced rows. Exemplary of other patterns which may be used is the so-called circular flood pattern in which the injectionsystem comprises a central injection well and the production system comprises a plurality of production wells spaced radiallyabout the injection well. Of course, the injection and production systems each may consist of only a single well in which case the recovery zone as defined by the spaced injection and production wells will be the portion of the reservoir underlying a generally elliptical area between these wells which is subject to the displacing action of the injected flooding medium. The above and other patterns are well known to those skilled in the art and for a more detailed description of such patterns, as well as flood techniques in general, reference is made to Uren, L. C., PETROLEUM PRODUCTION ENGINEERING OIL FIELD EXPLOITA- TION, 2nd Edition, McGraw-I-Iill Book Company Inc., New York and London, 1939, and more particularly to the section entitled ,The Water Flooding Process," appearing at pages 444-459.

It also will be recognized that the invention may be carried out utilizing oneor more dually completed injection-production wells of the type, for example, disclosed in U.S. Pat. No. 2,725,106. This arrangement sometimes may be utilized to advantage in a relatively thick oil reservoir in which it may be desirable to displace the oil in the reservoir upwardly and recover such oil from the upper portion of the reservoir. In this instance, the injection system normally would comprise the lower completion interval of one or more dually completed wells of the type described in the aforementioned U.S. Pat. No. 2,725,I()6, and the production system would comprise the upper completion interval of one or more of such wells. In this case, of course, the recovery zone would be that portion of the reservoir subject to the displacing action of the flooding medium as it moves upward through the reservoir.

The present invention is normally'carried out in a recovery I zone which has previously undergone secondary recovery by means of waterflooding techniques although it may be carried out as an initial recovery operation. In carrying out the present invention, gas, e.g., air, nitrogen, flue gas, etc., or combinations thereof, is injected through an injection system into the recovery zone. The purpose of this gas injection is to establish a free-gas phase in the recovery zone. The amount of gas required will vary due to size and the physical characteristics of the recovery zone. For example, a volume of gas equal to as little as 2 percent of the recovery zone pore volume may be sufficient if the recovery zone is essentially homogeneous while a volume of gas in excess of 20 percent of the recovery zone pore volume may be required if the recovery zone contains high permeability streaks. Preferably, the production wells are left open and production continues during the gas injection step. If the recovery zone is small, gas may break through at the production wells but normally the recovery zone is such that a free-gas phase will only be formed in a portion of the recovery zone which surrounds the injection system.

After a free-gas phase is established in the recovery zone, a simultaneous flood of gas-water is carried out. The term water as used here can be any flooding liquid normally used in waterflooding operations, e.g., fresh water, brines, water with or without additives such as thickeners, emulsions, etc. A mixture of gas (e.g., air) and water in which there is 25 to 75 percent as much gas, at reservoir conditions, as water, is used for the simultaneous gas-water injection step. That is to say, the volume of water in the total injected fluid may vary from 25 to 75 percent, both the gas and water volumes being measured at reservoir conditions. Again, preferably the production wells are left open during the simultaneous flooding step.

In carrying out the simultaneous injection step, the actual method of injecting the gas-water mixture forms no part of the present invention. One technique which may be used is to pack off the borehole adjacent the recovery zone by conventional methods and run two lines from the surface 'into the packed-off area. Gas and water are then supplied simultaneously through separate lines and allowed to mix as they are both injected into the recovery zone through the packed-off area, or the first few feet around the borehole may be used as a mixing zone, as briefly described above.

Simultaneous injection of gas and water continues until a volume equal to from 2 to 20 percent of the pore volume of the recovery zone has been injected. The desired volume of the gas-water mixture will normally correspond roughly to the volume of gas injected during the initial gas step. That is, if 2 percent pore volume of gas is injected during the initial step, then 2 percent pore volume of gas-water mixture would follow. The initial step of gas injection followed by a simultaneous gas-water injection step is considered a complete cycle and the alternate steps are repeated, in order, until at least one cycle reaches the production system. Normally, the process will be repeated until production drops below an economical level and may include several cycles.

By first establishing a free-gas phase within the recovery zone, it is theorized that the large pores and the interior portions of the pores are filled with gas. When this followed with a simultaneous gas-water flood, the gas maintains this filled portion and the water is forced to flow nearer the surface of the rock interfaces thereby displacing the oil toward the production system. Also, since the larger pores are filled with gas, it is believed that water is forced into the smaller pore spaced to displace oil therefrom thereby increasing the ultimate recovery from a reservoir.

What I claim Is:

1. A process for recovering petroleum from a recovery zone of a subterranean reservoir having an injection system and a production system extending into said recovery zone, said process comprising:

injecting gas into said recovery zone through said injection system to establish a free-gas phase in at least a portion of said recovery zone;

injecting gas and water simultaneously into said recovery zone through said injection system;

producing fluids from said recovery zone through said production system; and

repeating said alternate gas and simultaneous gas-water injection steps until production drops to an uneconomical level.

2. A process for recovering petroleum from a recovery zone of a subterranean reservoir having an injection system and a production system extending into said recovery zone, said process comprising:

injecting gas into said recovery zone through said injection system to establish a free-gas phase in at least a portion of said recovery zone;

said gas is injected in an amount equal to from 2 to 20 percent of the pore volume of said recovery zone;

injecting gas and water simultaneously into said recovery zone through said injection system; and

producing fluids from said recovery zone through said production system.

3. The process of claim 2 wherein:

said combined volume, measured at reservoir conditions, of gas and water injected simultaneously is equal to from 2 to 20 percent of the pore volume of said recovery zone.

4. The process of claim 2 wherein:

said gas-water mixture used in said simultaneous injection step comprises 25 to 75 percent water to gas, both measured at reservoir conditions.

5. A process for recovering petroleum from a recovery zone of a subterranean reservoir having an injection system and a production system extending into said recovery zone, said process comprising:

a. injecting gas into said recovery zone through said injection system while producing fluids from said recovery zone through said production system;

b. continuing injection of gas until a volume equal to 2 to 20 percent of the recovery zone pore volume has been injected; and

c. injecting gas and water simultaneously into said recovery Zone through said injection system while producing fluids from said recovery zone, said gas-water mixture being from 25 to 75 percent water to gas, both measured at reservoir conditions.

6. The process of claim 5 wherein:

said combined volume, measured at reservoir conditions, of said gas and water injected simultaneously is equal to the same volume as that of said gas injected in step (a).

7. The process of claim 6 including:

repeating said alternate gas and simultaneous gas-water injection steps until production drops to an uneconomical level. 

2. A process for recovering petroleum from a recovery zone of a subterranean reservoir having an injection system and a production system extending into said recovery zone, said process comprising: injecting gas into said recovery zone through said injection system to establish a free-gas phase in at least a portion of said recovery zone; said gas is injected in an amount equal to from 2 to 20 percent of the pore volume of said recovery zone; injecting gas and water simultaneously into said recovery zone through said injection system; and producing fluids from said recovery zone through said production system.
 3. The process of claim 2 wherein: said combined volume, measured at reservoir conditions, of gas and water injected simultaneously is equal to from 2 to 20 percent of the pore volume of said recovery zone.
 4. The process of claim 2 wherein: said gas-water mixture used in said simultaneous injection step comprises 25 to 75 percent water to gas, both measured at reservoir conditions.
 5. A process for recovering petroleum from a recovery zone of a subterranean reservoir having an injection system and a production system extending into said recovery zone, said process comprising: a. injecting gas into said recovery zone through said injection system while producing fluids from said recovery zone through said production system; b. continuing injection of gas until a volume equal to 2 to 20 percent of the recovery zone pore volume has been injected; and c. injecting gas and water simultaneously into said recovery zone through said injection system while producing fluids from said recovery zone, said gas-water mixture being from 25 to 75 percent water to gas, both measured at reservoir conditions.
 6. The process of claim 5 wherein: said combined volume, measured at reservoir conditions, of said gas and water injected simultaneously is equal to the same volume as that of said gas injected in step (a).
 7. The process of claim 6 including: repeating said alternate gas and simultaneous gas-wAter injection steps until production drops to an uneconomical level. 